<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="i-dont-exist.css">
<script>
var xhr = new XMLHttpRequest;
xhr.open('POST', 'post-echo.pl', true);
xhr.onreadystatechange = function () {
  if (xhr.readyState != 4)
      return;
  console.log(xhr.responseText);
}
xhr.send("Post - ECHO SUCCESS!");
</script>
</head>
</html>
